There has been widely used a geared motor as an actuator for operating various control systems. For instance, in a vehicle driven by an internal combustion engine, the geared motor has been used as an actuator to make an operation of changing an automatic transmission between a two wheel drive state and a four wheel drive state, an actuator to operate a clutch of the automatic transmission or an actuator to operate various valves such as an exhaust control valve or a throttle valve.
As shown in JP2001-45710A, the geared motor comprises an electric motor, a reduction gear having an input portion connected to a rotational shaft of the electric motor and an output shaft provided on an output stage gear of the reduction gear to be connected directly or through an appropriate connection member to an operated portion of a load as the control object of the output shaft.
The electric motor also comprises a position sensor to detect information on a rotational angle position of the output shaft and provide the information for a control device because it is used for controlling the position of the load. As shown in JP2001-45710A, the position sensor generally comprises a potentiometer having an input shaft connected directly to the output stage gear of the reduction gear.
In the geared motor having the output stage gear connected directly to the input shaft of the position sensor, when the central axis of the output shaft is slightly inclined relative to that of the input shaft of the position sensor due to a radial load applied from the load to the output shaft, the radial load is applied from the output shaft to the input shaft of the position sensor whereby bearings in which the input shaft of the position sensor is supported are worn out so that the input shaft of the position sensor will get loosely supported or be broken.
Especially, if the connection point of the output shaft and the work load is located at a position shifted relative to the center axis of the output shaft, which will occur when the output shaft is connected through a lever to the work load, for instance, great radial load would be applied to the output shaft whereby the aforementioned problems tend to occur.
In JP11-23207A, it is disclosed a rotational displacement detector for detecting by a potentiometer a rotational displacement of a rotational body as an objective to be detected. It is shown in this document that the rotational body and an input shaft of the potentiometer are connected to each other through a bush of resilient material such as rubber.
In the rotational displacement detector shown in JP11-23207A, the input shaft of the potentiometer has a leading end of D-shaped cross section, the rotational body has the bush of resilient material fitted into a hole in an axis portion of the end thereof, and the input shaft of the potentiometer and the rotational body are connected by press-fitting the D-shaped cut portion of the leading end of the input shaft of the potentiometer into the D-shaped cut hole (hole of D-shaped cross section) in the center of the bush. There is used a member having only a portion engaging the hole in the axis portion of the rotational body for the bush interposed between the input shaft of the potentiometer and the rotational body.
With the construction aforementioned, when the radial load is applied to the output shaft, the radial load applied from the output shaft to the input shaft of the potentiometer can be absorbed by the bush of resilient material so that great radial load is prevented from being applied to the input shaft of the potentiometer whereby the breakage of the potentiometer can be prevented.
It will be considered that the geared motor to which the invention is applied may have such a bush of resilient material fitted into an axial hole of the output stage gear as suggested by the JP11-23207A whereby the input shaft of the position sensor and the output stage gear are coupled through the bush.
However, if the input shaft of the position sensor and the output stage gear are coupled through the bush having only the portion fitted into the axial hole in the output stage gear, it will be hard to increase the mechanical strength of the bush whereby the bush tends to be twisted by an unnegligible degree when the rotational body begins to rotate or stops. Thus, there apparently occurs a problem of reducing the position detection precision of the output shaft.
More particularly, if the bush coupling the rotational body and the input shaft of the position sensor is possibly twisted, there occurs a difference between the rotational angle of the output shaft and that of the input shaft of the position sensor. As a result, there occurs hysteresis in the characteristic of detection of the displacement of the rotational body by the position sensor, which reduces the detection precision of the displacement of the rotational body.
It is required to have a fully high precision of the angular position of the output shaft in order to mount the output shaft on an axis portion of the output stage gear and therefore, if the input shaft of the position sensor is connected to the output sides gear through the bush, it is required to mount the output shaft in a predetermined position relation to the bush coupling the output shaft to the input shaft of the position sensor. If there is used the bush having only the portion fitted into the axial hole in the output stage gear, in a state where the gear is positioned by inserting into a non-circular hole (normally D-shaped) in the center portion of the bush in order to fit the input shaft of the position sensor, it will be considered that the output shaft is press-fitted into the axial hole in the gear.
In this manner, if the gear is positioned by inserting the positioning tool into the central hole in the bush of resilient material, the positioning tool cannot be tightly inserted into the central hole in the bush in order to prevent the deformation of the bush. Thus, there cannot be avoided some clearance between the hole in the bush and the positioning tool. In this manner, if there occurs an error of the angle position due to the clearance between the hole in the bush and the positioning tool, even though the error is slight, there appears the greater error of the angular position of the output shaft in the side of the outer periphery thereof. Therefore, it is hard to obtain the precision of the angle position of the output shaft if the central hole in the bush is used as a positioning reference.